The present invention generally relates to magnetoelectronics, and more particularly to material composition and fabrication techniques for magnetoelectronics elements.
Magnetoelectronics, spin electronics, and spintronics are synonymous terms for the use of effects predominantly caused by electron spin. Magnetoelectronics is used in numerous information devices, and provides non-volatile, reliable, radiation resistant, and high-density data storage and retrieval. The numerous magnetoelectronics information devices include, but are not limited to, magnetic random access memory (MRAM), magnetic sensors and read heads for disk drives.
Typically, a magnetoelectronic device, such as a magnetic memory element, has a structure that includes multiple ferromagnetic layers separated by at least one non-magnetic layer. Information is stored as directions of magnetization vectors in the magnetic layers. Magnetic vectors in one magnetic layer, for instance, are magnetically fixed or pinned, while the magnetization direction of the other magnetic layer is free to switch between the same and opposite directions that are called xe2x80x9cparallelxe2x80x9d and xe2x80x9cantiparallelxe2x80x9d states, respectively. In response to parallel and antiparallel states, the magnetic memory element represents two different resistances. The resistance has minimum and maximum values when the magnetization vectors of the two magnetic layers point in substantially the same and opposite directions, respectively. Accordingly, a detection of change in resistance allows a device, such as an MRAM device, to provide information stored in the magnetic memory element. The difference between the minimum and maximum resistance values, divided by the minimum resistance is known as the magnetoresistance ratio (MR).
The physical structure of these magnetic elements typically includes very thin layers, some of which are tens of angstroms thick or less. The performance of the magnetic element is sensitive to condition of the surface on which the magnetic layers are deposited. Accordingly, it is generally desirable to make as flat a surface as possible in order to prevent the operational characteristics of a magnetic element from degrading.
During typical magnetic element fabrication, such as MRAM element fabrication, which includes metal films grown by sputter deposition, evaporation, or epitaxy techniques, the film surfaces are not absolutely flat but instead tend to exhibit surface or interface roughness. This roughness of the surfaces and/or interfaces of the ferromagnetic layers is the cause of magnetic coupling between the free ferromagnetic layer and the other ferromagnetic layers, such as the fixed layer or pinned layer, which is known as xe2x80x9ctopological couplingxe2x80x9d or xe2x80x9cNxc3xa9el""s orange peel coupling.xe2x80x9d Such coupling is generally undesirable in magnetic elements because it can create an offset in the response of the free layer to an external magnetic field.
The roughness can also have undesirable effects on the electrical properties of the device by affecting the quality of the interfaces between the magnetic layers and the non-magnetic spacer layer. In a typical tunnel junction application, such roughness may also lead to variations in the thickness of the spacer layer and, correspondingly, to variations in the resultant tunneling current.
A magnetic structure is known as xe2x80x9cbottom pinnedxe2x80x9d when the fixed layer is formed before the spacer layer, and the free layer is formed after the spacer layer. In such a bottom-pinned structure the antiferromagnetic (AF) pinning layer is contained in the bottom magnetic electrode. Conventional bottom-pinned magnetic tunnel junctions (MTJs) and spin valve structures typically use seed and template layers to produce an oriented, crystalline AF layer for strong pinning. The bottom electrode of a typical bottom-pinned MTJ structure includes stacked layers of Ta/NiFe/FeMn/NiFe, which is followed by the AlOx tunnel barrier, and a top electrode that typically includes a free layer of NiFe, where the Ta/NiFe seed/template layers induce growth of large and highly oriented FeMn crystallites in the FeMn layer and the pinned magnetic layer. Such highly oriented polycrystalline layers may also be described as being xe2x80x9cstrongly textured.xe2x80x9d This strongly textured FeMn layer provides for strong pinning of the NiFe layer below the AlOx tunnel barrier. The FeMn layer, or other oriented polycrystalline AF layer typically produces a roughness that can cause an increase in the undesirable Nxc3xa9el coupling between the pinned NiFe layer and the top free NiFe layer as well as variations in the tunneling current.
In practical MTJ elements, the bottom electrode is generally formed upon a base metal layer that provides a relatively low resistance contact to the junction. The base metal layer is typically polycrystalline and tends to grow in a columnar-like fashion. This can produce a roughness that, in turn, propagates into the bottom electrode and produces roughness at the spacer layer interfaces, resulting in an increase in undesirable magnetic and electrical properties. The roughness propagated from the base metal layer and the bottom electrode is additionally disadvantageous because it limits the minimum tunnel barrier thickness that can be achieved while retaining high MR and device resistance that scales inversely with junction area.
In order to reduce the roughness of the layers and the layer interfaces, various types of non-crystalline or amorphous materials have been developed for use in the various layers of the MTJ stack. Since the non-crystalline or amorphous materials lack the crystal boundaries and sharp features of other materials, the tunnel barrier resulting from the layers with the amorphous materials can provide for better device performance. However, while the use of amorphous materials can be desirable, this requirement dramatically limits the choice of alloys for the magnetic layers to those few that are amorphous. In addition, a thin layer of amorphous magnetic material formed on a crystalline pinning layer tends to replicate at least some of the surface roughness of the underlying surface. This leads to diminished value for the layer of amorphous material.
Accordingly, it is desirable to provide materials and methods for consistently creating smooth layer interfaces in MTJ stacks, thereby enhancing the performance of the magnetic elements formed thereby. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings.